Method and device for transmitting data having a variable bit length

ABSTRACT

A method for serially transmitting data in a bus system having at least two bus users, which exchange data frames over the bus, the bus users deciding which data frames they receive, as a function of an identifier, the data frames having a logic structure according to the CAN standard, ISO 11898-1, the temporal bit length (L 1,  L 2 ) inside of a data frame being able to assume at least two different values, the temporal bit length (L 1 ) for at least one first, specified or specifiable region within the data frame meeting the requirements of the CAN standard, ISO 11898-1, the temporal bit length (L 2 ) in at least one second, specified or specifiable region being reduced in comparison with the first region, and a change in the temporal bit length being signaled by the sender, using an identification ( 310 ) contained in the same or one of the preceding data frames.

FIELD OF THE INVENTION

The present invention relates to a device, a method and an interface fortransmitting data between at least two users of a CAN bus system, thetemporal duration of the transmitted bits inside of a data frame beingable to assume at least two different values.

BACKGROUND INFORMATION

The controller area network and an expansion of the CAN referred to as atime-triggered CAN (TTCAN) are known from the published patentapplication DE 100 00 305 A1. The media access control method used inthe CAN is based on bitwise arbitration. In bitwise arbitration, severaluser stations may transmit data simultaneously over the channel of thebus system without this interfering with the data transmission. Uponsending a bit over the channel, the user stations may also ascertain thelogic state (0 or 1) of the channel. If a value of the sent bit does notcorrespond to the ascertained logic state of the channel, then the userstation terminates the access to the channel. In the CAN, the bitwisearbitration is normally carried out in an arbitration field within adata frame to be transmitted over the channel.

After a user station has completely sent the arbitration field to thechannel, it knows that it has exclusive access to the channel. Thus, theend of the transmission of the arbitration field corresponds to abeginning of an available interval, within which the user station mayuse the channel exclusively. According to the protocol specification ofthe CAN, other user stations may not access the channel, that is, senddata to the channel, until the sending user station has transmitted achecksum field (CRC field) of the data frame. Thus, an end time of thetransmission of the CRC field corresponds to an end of the availableinterval.

The bitwise arbitration allows non-destructive transmission of the dataframe over the channel to be achieved. This produces effective real-timecharacteristics of the CAN, whereas the media access control methods, inwhich the data frame sent by a user station may be destroyed, due to acollision with a further data frame sent by another station during thetransmission over the channel, have a markedly poorer real-timeperformance, since the collision and the re-transmission of the dataframe rendered necessary by it result in a delay in the datatransmission.

The protocols of the CAN and of its expansion, TTCAN, are particularlysuited for transmitting short messages under real-time conditions.However, if larger data blocks are to be transmitted over a CAN domain,then the relatively low bit rate of the channel becomes a limitingfactor. In order to ensure the correct functioning of the bitwisearbitration, a minimum time period dependent on, in particular, theextent of the bus system, the signal propagation speed on the channel,and intrinsic processing times in the interface modules of the bususers, must be maintained for the transmission of a bit, since duringthe arbitration, all of the bus users must have a uniform picture of thebus state (0 or 1) and equal access to the bus state. Thus, the bit ratecannot easily be increased by reducing the duration of the individualbits.

Nevertheless, in order to be able to transmit sufficiently rapidly arelatively large data block, which is necessary for the programming of acontrol unit, via a communications interface actually provided forconnecting to a CAN domain, DE 101 53 085 A1 provides that fortransmitting the data block, the communications interface be temporarilyswitched over into a different communications mode, in which bitwisearbitration is not carried out and, therefore, a relatively high bitrate is possible. However, in this connection, the communication withthe protocols of the CAN must be interrupted for a certain amount oftime. If, e.g., due to an error, the operation of the bus system cannotbe initiated in accordance with the CAN protocols, then the bus systemwill stop functioning. In addition, due to the transmission of arelatively large data block, the subsequent transmissions to be carriedout according to the CAN protocols are considerably delayed, which meansthat the real-time characteristics of the CAN are seriously degraded.Therefore, it is not practical to use this method both for programmingthe control unit at the end of a manufacturing process of a motorvehicle or of the control unit, and during operation of the vehicle.

German patent document DE 103 11 395 A1 discusses a system, which may beswitched over between CAN communication and an asymmetric, serialcommunications protocol and allows a higher transmission rate in theasynchronous mode. However, this protocol is outside of the CANstandard.

German patent document DE 103 40 165 A1 discusses improvedsynchronization between sensors and actuators within a CAN network. Thisreduces the latency times, but the transmission rate is not increased.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method, as well as adevice and an interface, by which larger quantities of data may betransmitted relatively rapidly in a CAN network and real-time conditionsmay be maintained during the transmission of messages over the bus. Thisobject is achieved by the data transmission method having the featuresof Claim one, as well as by the device and interface described in theindependent claims.

The above-described object of the present invention is achieved in thatfor a limited period of time, in particular, after arbitration hasoccurred, the bit length, i.e., the temporal duration of the bits,within a data frame is reduced in comparison with the value used for thearbitration. In other words, for a limited section inside of the datafield of the transmitted data frame, the clock-pulse rate of the bus isincreased in comparison with the base clock-pulse rate. In thisconnection, the clock-pulse rate may also assume, in principle, severaldifferent, increased values, and the instants for switching over betweenthese values may lie anywhere within the data frame. In completelygeneral terms, the clock-pulse rate may assume any characteristic,provided that the bus users have a uniform understanding of thischaracteristic and may thus interpret the information contained in thedata frames in a standard manner.

The fact, that a data frame has, in at least a subsection, a bit lengthchanged according to the present invention, must be communicated to thereceiver, in order that it can interpret the sent data correctly. Themessage to the receiver takes place via an identification. In thisconnection, it may be advantageous to use stored values dependent on theidentification, for the bit length and the times of the change in thebit length. An example of this would be an identification via a set bitstill available inside of the header region of the data frame; for oneof the two possible values of this bit, a bit length reduced by a factorset for all of the bus users being used, for example, in the completedata field.

Inside of the identification or at another fixed position inside of thedata frame, another option is to transmit information regarding the bitlength and/or the times of the change in the bit length to the receiver.For example, a reduction factor or reduction divisor for the bit lengthcould be transmitted as a whole number in the first three bits of thedata field, and the bit length multiplied by the factor could be used,after a set switchover time, for the remaining duration of the datafield.

In one further advantageous specific embodiment, the identification issent in a preceding data frame, and in this manner, it is indicated toat least one bus user that a subsequent data frame sent to it will havea reduced bit length.

It is further advantageous when at least an SOF bit and arbitrationfield of the data frame have a bit length, which corresponds to the baseclock-pulse rate of the bus. In this case, depending on the result ofthe arbitration, the bit length may be selectively reduced in only thesender and receiver of the currently transmitted data frame, and energy,which would be necessary for producing the corresponding increase in thesampling frequency in all of the bus users, is saved. The other bususers, which do not switch over their bit length, are then temporarilyunable to participate in the communication.

In order to use the devices, which execute the method of the presentinvention, in both bus systems whose users continually maintain a bitlength corresponding to the base clock-pulse rate, and in bus systemswhose users may operate with a bit length reduced according to thepresent invention, it is further advantageous to design the devices tobe switchable, for example, via a suitable input. The devices operatingaccording to the present invention may then be used in a flexible mannerin old and new networks.

In order to prevent the bits transmitted with a reduced bit length fromcausing errors in bus users that do not switch over their bit length, orto prevent an erroneous resynchronization onto the edges of the bitstransmitted with a reduced bit length, it may be advantageous that inthe method described, in the case of one or more bus users, inparticular, in the bus users that do not switch over their bit length,the resynchronization is discontinued at least during the transmissionof data having a reduced bit length. For example, a bus system couldhave full nodes and Eco nodes as bus users, the full nodes beingconfigured to switch over the bit length, using suitable devices, suchas higher-quality oscillators, while the Eco nodes are only able to usethe normal bit length. If a full node sends a message having a shortenedbit length, then, for example, the Eco nodes discontinue theircommunication as of the last long bit and also no longer carry out a bitresynchronization.

It is possible to render the suspension of the resynchronizationdependent on the detection of an imminent reduction in the bit length,for example, using a dependence on the identifier of the data frame, oron the result of the arbitration and/or on a suitable identification.The Eco nodes re-initiate the communication, when they observe arecessive bus state for a specifiable time period, for example, for tenslow bit lengths, which corresponds to an end-of-frame having threesubsequent intermission bits. Then, an inaccurate, inexpensive andenergy-saving oscillator is sufficient for the corresponding detection.

In the following, the present invention is set forth and explained infurther detail in view of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a CAN bus system from the related art, having several usersthat may exchange data frames over the bus.

FIG. 2 a schematically shows the structure of a data frame according tothe CAN standard, ISO 11898-1.

FIG. 2 b schematically shows the structure of a data frame, includingthe partitioning into regions of variable bit length according to thepresent invention.

FIG. 3 illustrates an example of the temporal position of theidentification within the data frame, by which the necessary informationabout the change in the bit length is communicated to the receiver.

FIGS. 4 a and 4 b show different options for distributing the necessaryinformation about the change in the bit length between data frames andbus users of the present invention.

DETAILED DESCRIPTION

In the following, exemplary embodiments of the method and the device ofthe present invention are described. These concrete examples are usedfor explaining the implementation but do not limit the scope of theinventive idea.

FIG. 1 shows a CAN bus 100 from the related art, including several users110, 120, 130, 140 as illustrated in FIG. 1. Data frames are exchangedbetween the users in accordance with the CAN standard, ISO 11898-1. Thebus has, for example, a clock-pulse rate of 500 kBaud; that is to say,the bit length is, in this example, 2 μs.

FIG. 2 a shows the schematic structure of a data frame 200, which may betransmitted over bus 100. In principle, the data frame may be subdividedinto a header region 201 (normally made up of the fields “start offrame,” “arbitration field,” and “control field”), as well as a datafield 202 and an end region 203 (normally made up of a CRC field, ACKfield and “end of frame”). In the case represented here, thetransmission of the data field begins at time t5, and the transmissionof the end region begins at time t6. The clock-pulse rate is constantfor the entire data frame.

FIG. 2 b shows an example of the different structure of a data frame 210according to the present invention, including header region 211, datafield 212 and footer region 213. Additionally represented are times t3and t4, which indicate the beginning and the end of switching over thebit length. In the case represented here, the clock-pulse rate isincreased, for example, by a factor of 4 for the entire data field, andthe bit length is reduced by the inverse factor to 0.5 μs. Accordingly,time t3 coincides with the beginning of the data field of the data framet5, and time t4 coincides with the beginning of the end region of thedata frame t6. However, in a different embodiment, times t3 and t4 mayalso be situated at other positions, for instance, inside of the datafield.

FIG. 3 shows, once more, the structure of a data frame 210 of thepresent invention. The position of identification 310 of the presentinvention inside of the header region of the data frame is alsoillustrated by way of example. In the embodiment shown here, one or morebits inside of the header region could be used for the identification.For example, a reserved bit could be used as an identification of thebit length reduced by a factor of four. In this case, the indicatedtimes t1 and t2 would be the start and end of the reserved bit, and asbefore, times t3 and t4 correspond to the beginning and end of theswitching-over of the bit length.

A special case of the identification of the present invention is theoption of setting the reduced bit length used for the transmission, as afunction of the identifier of the data frame within the scope of thedefinition of the CAN matrix. If, while setting it, the potentialsenders and receivers of the specific data frame are known, then it isalso known if all of the senders and receivers are configured for amessage having the given identifier for using the reduced bit length,that is, if they are so-called “full nodes,” or if they are notconfigured for this, that is, if they are “Eco nodes.” Depending onthis, it may then be established that data frames, which have particularidentifiers or are made up of a group of identifiers, in particular, theones that are only exchanged between full nodes, use a reduced bitlength in a stipulated region. This is then communicated to the specificreceivers by only the identifier itself.

A further option that is, however, not explained here in further detailis to send identification 310 in a preceding data frame and, in thismanner, to indicate to at least one bus user, that a subsequent dataframe sent to it will have a reduced bit length. In this case, theidentification does not have to be sent along in each data frame of thepresent invention, but only one time or sporadically.

FIGS. 4 a and 4 b relate to the information content of theidentification. In this case, for different types of data frames, thereis the option of storing the information about the specificcharacteristic of the bit length inside of the data frame in the bususers, and transmitting only an information item regarding type withinthe scope of the identification. Alternatively, characteristicquantities, which describe the reduction in the bit length, may also besent with the identification.

In the case illustrated, there are only two types of data frames, andonly one bit of data frame 210 is used as identification 310, in orderto switch over between type A (e.g., data frame having a uniform bitlength according to the CAN standard, as in FIG. 2 a) and type B (e.g.,data frame having a bit length in the data field shortened by a factorof 4, as in FIG. 2 b). Eco bus users 405 and 406, which send and/orreceive data frames of the type A, know the standard bit length L1. Fullbus users 410 and 420, which send and/or receive data frames of the typeB, know the two values of the occurring bit lengths L1 and L2, theposition of the identification [t1, t2], and the starting and endingtimes t3 and t4 of the region having a reduced bit length, adapt theirsending and receiving characteristics in a suitable manner. In order tobe able to possibly interrupt the reception temporarily and switch offthe bit resynchronization, Eco users 405 and 406 additionally know theposition of the identification [t1, t2]. The dependence of the presentinvention's reduction in the bit length on the identifier of the dataframe, which was discussed in connection with FIG. 3 b, also requiresknowledge of the identifying region [t1, t2] in users 405 and 406; inthis case, the identifying region being implicitly in the identifier ofthe data frame.

Of course, it is also possible that only data frames of the type B areused on the bus and, accordingly, that all of the bus users are definedas full users and adjust their sending and receiving characteristics toa shortened bit length.

Another option is illustrated in FIG. 4 b. In this case, in addition toidentification 310, portions of the information about the change in thebit length may also be transmitted, in particular, the factor F, bywhich the data transmission rate of the bus is to be increased and thebit length is to be reduced. In this example, full bus users 430 and440, which send and/or receive data frames of the type B, know theposition of the identification [t1, t2] and the starting and endingtimes t3 and t4 of the region having a reduced bit length, calculate thechanged bit length as L2 =L1/F and adapt their sending and receivingcharacteristics in a suitable manner. For example, by using the firstthree bits of the data field, eight different factors F, such as thevalues between 1 and 8, could be transmitted, and the bit length couldbe correspondingly switched over at a defined time after reception ofthe factor, for example, at the fourth or fifth bit of the data field.

In the exemplary embodiments represented by FIGS. 4 a and 4 b, themethod may be implemented in such a manner, that a switchover of thesampling rate takes place exactly when a type-B data frame having areduced bit length is transmitted. In this case, it is necessary for theknowledge of the reduced bit length to have arrived at the receiving bususer in a timely manner. Thus, in particular, the identification must betransmitted prior to the region having a reduced bit length, thus, inthe example shown, temporally prior to the data field, either in thesame or a preceding data frame.

Furthermore, it is also possible that only some of the users, inparticular, only the specific sender and receiver, execute theswitchover of the sampling rate, when a type-B data frame having areduced bit length is transmitted.

To execute the method, full nodes have, for example, a higher-qualityoscillator, whereas for cost savings, a correspondingly more expensiveoscillator may be dispensed with in the Eco nodes. For the Eco nodes,the resynchronization may be switched off in the time frame in whichdata having a reduced bit length are transmitted, in order to preventincorrect resynchronization on edges of the bus signal, which may occurdue to the transmission of the data frames having a reduced bit length.In the Eco nodes, the receiving operation may also be completelyinterrupted, and, for example, an energy-saving quiescent state may beassumed, in which, for instance, voltage is not applied to some modulesof the Eco node. This switching-off and/or interruption may be rendereddependent on the detection of an imminent reduction in the bit length.The identification of the data frames according to the present inventionmay be utilized for this, that is, upon detection of an identification310, which signals a type-A data frame having a shortened bit length,the Eco bus users would switch off the resynchronization orcommunication for this data frame.

They re-initiate the resynchronization or communication, for example,when they observe a recessive bus state for a specifiable time period,for example, for ten slow bit lengths, which corresponds to anend-of-frame having three subsequent intermission bits.

It would also be possible for a classification of the data frameaccording to type (in the case represented by way of example, as type Aor type B) to be stipulated within the scope of the definition of theCAN matrix, depending on, for example, whether the sender and thereceiver of the specific data frame are full nodes or Eco nodes; and forthe resynchronization for the type-B data frames determined in thismanner to be switched off. To this end, the corresponding address listsor filters in the bus users would have to be provided with thecorresponding information item regarding the type.

1-18. (canceled)
 19. A method for serially transmitting data in a bussystem having at least two bus users, which exchange data frames overthe bus, the bus users deciding which data frames they receive, as afunction of an identifier, the method comprising: reducing the temporalbit length in at least one second, specified or specifiable region incomparison with the first region; and signaling a change in the temporalbit length by the sender, using an identification contained in the sameor one of the preceding data frames; wherein the data frames have alogic structure according to the CAN standard, ISO 11898-1, wherein thetemporal bit length inside of a data frame may assume at least twodifferent values, the temporal bit length for at least one first,specified or specifiable region within the data frame meeting therequirements of the CAN standard, ISO 11898-1.
 20. The method of claim19, wherein the identification is situated inside the first specified orspecifiable region of the indicated data frame.
 21. The method of claim19, wherein the first region includes at least the SOF bit and thearbitration field.
 22. The method of claim 20, wherein theidentification is derived from portions or the entirety of theidentifier of the indicated data frame.
 23. The method of claim 20,wherein the identification is carried out, using a single bit inside ofthe first region.
 24. The method of claim 19, wherein the temporal bitlength may assume exactly two different values.
 25. The method of claim19, wherein in addition to the identification, at least one informationitem is transmitted, from which the bit length to be used in the atleast one second region is derivable.
 26. The method of claim 19,wherein the first region includes several bits of the data field, andthe at least one information item is transmitted in these bits.
 27. Themethod of claim 19, wherein the identification is sent in a precedingdata frame, and using the identification, at least one bus user isinformed that a subsequent data frame sent to it will have a reduced bitlength.
 28. The method of claim 19, wherein a first group of bus usersimplements a reduction in the bit length as a function of theidentification, and a second group of bus users discontinues the bitresynchronization and/or interrupts the receiving operation and/orassumes a quiescent state as a function of the identification.
 29. Themethod of claim 28, wherein the first group of bus users includes all ofthe bus users, which are designed as full nodes and are thereforeconfigured for using the shortened bit length.
 30. The method of claim28, wherein the first group of bus users includes all of the bus users,which are designed as full nodes and are therefore configured for usingthe shortened bit length, and which are either senders or receivers ofthe data frame just transmitted.
 31. A device for serially transmittingdata in a bus system having at least two bus users, which exchange dataframes over the bus, the bus users deciding which data frames theyreceive, as a function of an identifier, comprising: a reducingarrangement to reduce a temporal bit length in at least one second,specified or specifiable region in comparison with the first region; anda signaling arrangement to signal, by the sender, a change in thetemporal bit length, using an identification contained in the same orone of the preceding data frames; wherein the data frames have a logicstructure according to the CAN standard, ISO 11898-1, wherein devicesare provided for switching over the temporal bit length inside of a dataframe between at least two different values, the temporal bit length forat least one first, specified or specifiable region within the dataframe meeting the requirements of the CAN standard, ISO 11898-1.
 32. Thedevice of claim 31, wherein devices are provided for executing a datatransmission method according to one of claims 2 through
 9. 33. A devicefor serially transmitting data in a bus system having at least two bususers, which exchange data frames over the bus, the bus users decidingwhich data frames they receive, as a function of an identifier,comprising: a signaling arrangement to signal, by the sender, a changein the temporal bit length by the sender, using an identificationcontained in the same or one of the preceding data frames; and devicesfor at least one of discontinuing the bit resynchronization andinterrupting the receiving operation and/or assuming a quiescent stateas a function of the identification; the data frames having a logicstructure according to the CAN standard, ISO 11898-1.
 34. The device ofclaim 33, wherein the bit resynchronization is discontinued and/or thereceiving operation is interrupted and/or a quiescent state is assumed,if, in light of the identification, it is detected that in at least tworegions, the data frame containing the identification has at least twodifferent values of the bit length.
 35. The device of claim 34, whereinthe bit resynchronization is discontinued and/or the receiving operationis interrupted and/or a quiescent state is assumed, if, in light of theidentifier, it is detected that the data frame indicated by theidentifier does not have to be read in by the device.
 36. The device ofclaim 31, wherein the device is manufactured to be switchable by aninput signal and may be switched over between a behavior according toCAN standard, ISO 11898-1, and the behavior modified according to thepresent invention.